1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor and, more particularly, to a method of manufacturing a semiconductor optical device having at least one optical axis alignment key.
2. Description of the Related Art
Semiconductor optical devices include a light source for generating light and a light-receiving device for receiving the light. A semiconductor laser is typically used as the semiconductor light source and includes an optical device that is aligned with a planar lightwave circuit (PLC). A passive alignment method and an active alignment method are mainly used in order to align the optical axes of the semiconductor optical device and the PLC.
According to the active alignment method, optical axes are aligned based on a comparison between the amount of the light generated in a semiconductor laser requiring the optical axis alignment and the amount of the light transmitted through the PLC. The active alignment method delivers a precise optical axis alignment, but requires a relatively longer processing time, which is not suitable for the mass-production thereof.
According to the passive alignment method, predetermined types of alignment keys are marked at the optical axis alignment position of a semiconductor laser and PLC. The passive alignment method is considered easier to align optical axes.
FIG. 1 is a cross-sectional view showing a conventional semiconductor optical device. As shown, the semiconductor optical device 100 includes a semiconductor substrate 110, active layers 120a and 120b, clads 121a and 121b having multi-layered well-shaped structures (formed through etching on the semiconductor 110 to yield a mesa structure), current insulating layers 130 and 140, an electrode 150, and first and second alignment keys 170 and 160.
The active layers 120a and 120b and the clads 121a and 121b are grown sequentially on the semiconductor substrate 110 and then etched to have a mesa structure. To achieve this, a mask (not shown) is positioned above the clads 121a and 121b. Then, the clads 121a and 121b are mesa-etched into a first alignment key 170 and an optical area 180 according to alignment patterns (not shown) formed in the mask and optical patterns (not shown). The optical area 180 performs optical functions of receiving and emitting light.
The first alignment key 170 serves as a reference for forming the second alignment keys 160, and the second alignment key 160 is spaced apart from the first alignment key 170 by a predetermined distance.
The current insulating layers 130 and 140 are formed at a location at which at least two semiconductor layers are mesa-etched. Further, the current insulating layers 130 and 140 may be formed by sequentially stacking a p-type semiconductor layer and an n-type semiconductor layer. After the current insulating layers 130 and 140 are grown, the optical device is flattened.
The electrode 150 is formed on the insulating layers 130 and 140, the first alignment key 170, and the optical area 180. The electrode 150 transfers current for driving the optical area 180.
The second alignment keys 160 are formed on the electrode using the first alignment key 170 as a guide. As such, the second alignment keys 160 are used to align the optical axis of the semiconductor optical device 100 with the optical axis of an optical device, such as a PLC. The optical axis alignment method using the second alignment keys 160 is referred to as a passive alignment method or a flip-chip bonding method. Thus, the second alignment keys 160 are called flip-chip bonding keys and may be made of a metal thin film or dielectric materials.
FIG. 2 is a photograph showing a portion of a semiconductor optical device manufactured according to the prior art as described above. An allowance for error in a general optical axis alignment is ±1 μm.
However, the border of the first alignment key becomes imprecise, as shown in FIG. 2, when undergoing a flattening and re-growing process of the semiconductor layer. Accordingly, when forming the second alignment key, the border of the first alignment key may cause the second alignment key to have an alignment error beyond the allowance. As a result, the optical axis alignment process is affected.